1. Field of the Invention
The present invention relates to a method for applying afterpressure on injection molding compounds, particularly PET compounds, in an injection molding machine including an injection mold and a screw connected to the injection mold through supply ducts. The present invention further relates to an apparatus for carrying out the method.
2. Description of the Related Art
After an injection mold has been filled with thermoplastic material, cooling of the material results in contraction or shrinkage of the molded article. This leads to non-uniform wall thicknesses and changes in the material structure because of the different temperatures in the molded article. For reducing the effects of the shrinkage of the volume, additional plastics material is conveyed into the injection mold and an afterpressure is simultaneously exerted on the material.
DE-OS 33 07 586 discloses a method of producing molded articles by injection molding, in which the material used for injection molding is collected in a storage unit arranged in the vicinity of the cylinder, the connection between the screw and the mold cavity is interrupted and afterpressure is applied to the stored material by means of a separate drive.
The disadvantage of this arrangement for injection molding articles of plastics material is the fact that the flow of material is separated from the principal supply flow and the material collected in the storage unit is conveyed subsequently. Storing a plastics material compound in a separate storage space has the following disadvantage: the material stored in the storage space will have different properties than the principal material during the various injection molding cycles. Since the material in the storage unit is not completely exchanged during each injection procedure, the stagnating material may be subjected to a thermal decomposition of the molecular structure.
DE-OS 35 16 179 discloses a method and an apparatus for the production of articles of thermoplastic materials by means of injection molding, in which a second storage space is provided between the screw and the injection mold, wherein the second storage space can be closed by a locking element and wherein an annular piston is provided for the second storage space which makes it possible to press additional thermoplastic material into the mold cavity by moving the entire screw unit.
This arrangement has the disadvantage that it is necessary to move the entire screw. In addition, the movement of such large units makes precise metering almost impossible.